Optimal Balance of Sun Exposure
UV exposure is known to have both detrimental and beneficial effect. Over exposure to sunlight can cause sunburn, skin aging, and skin cancer, whereas insufficient sunlight exposure can lead to vitamin D deficiency, which is associated with many health maladies. Therefore, there has been a growing consensus among many public health organizations that there needs to be a balance between the risks of having too much and the risks of having too little sunlight. However, the goal of achieving optimal balance of UV exposure remains elusive, since there is no definition of what is considered to be the optimal UV dosage, and consequently there is no quantitative means to assess whether a person's UV exposure is optimal or suboptimal. Therefore, there is a need to provide a system and method for quantitative evaluation the optimal balance of UV exposure, by taking into account both skin damage and vitamin D production effects of UV light. However, to develop a solution for this problem, there are several additional challenges that need to be overcome, as described below.
Safe Sun Exposure Time
One of the challenges is to quantify the damaging effect of UV exposure, which can be immediate and long-term. Sunburn is noticeable 3-4 hours after over exposure of UV light, peaking at around 24 hours. The long-term effects of excessive UV exposure include photo-aging, immunosuppression, and carcinogenicity. Since sunburn is a frequent detrimental effect on human skin, the CIE erythemal action spectrum is often recommended for use in assessing the skin damaging effect of UV radiation. The minimal erythemal dose (MED) is used to describe the erythemal potential of UV radiation, and 1 MED is defined as the effective UV dose that causes a perceptible reddening of previously unexposed human skin. The MED is known to vary between individuals, and is affected by many factors such as the skin type. Therefore, personalized UV irradiance dose monitoring is important. For a UV monitoring system, it is important to predict how long a person can safely stay exposed to sunlight before getting sunburn. The time to skin burn is a dynamic parameter. It is affected by the time varying intensity of UV irradiance. It also depends on the accumulated UV dose already received by the person. Moreover, it can be modified by the person's behaviors, such as seeking the shade, changing clothing coverage, applying the sunscreen, etc. To the best knowledge of the inventors, there is no existing solution for dynamic estimation of a person's safe sun exposure time.
Cutaneous Vitamin D Synthesis
Another challenge is to quantify the UV induced cutaneous vitamin D synthesis. Vitamin D, commonly known as the sunshine vitamin, actually functions as a hormone. Its main biologic function in people is to maintain serum calcium and phosphorus concentration within the normal range. Sufficient vitamin D is not only important for bone health, but also may decrease the incidence of diabetes, inhibit some autoimmune diseases such as multiple sclerosis, reduce the mortality from common cancers and cardiovascular diseases, among many other health benefits. However, studies have shown that many people do not go outside enough to meet their minimum vitamin D needs. In fact, vitamin D deficiency is now recognized as a pandemic. Therefore, there is a need for a solution to calculate daily vitamin D production resulting from UV exposure, and to provide user useful information to determine if vitamin D supplement is necessary, and if so, how much is necessary.
Effective Sun Protection Factor
A person's UV exposure is affected by the usage of sunscreen. The sun protection factor (SPF) is widely used to measure the effectiveness of a sunscreen in affording photoprotection to subjects exposed to sources of UV radiation that may produce cutaneous erythema. According to the U.S. Food and Drug Administration and international protocols, SPF shall be determined by using a prescribed sunscreen application thickness of 2.0 mg/cm2. However, many studies have shown that users typically apply much less sunscreen during leisure time than the suggested 2.0 mg/cm2, typically between 0.5 and 1.5 mg/cm2. Moreover, after a sunscreen is applied to the skin, it is commonly removed to a greater or lesser extend because of activities such as swimming, toweling, or excessive sweating and rubbing. Thus, frequent and regular reapplication of sunscreen is often recommended for users who experience extended period of sun exposure, but the optimal frequency and amount of sunscreen reapplication remain inconclusive. For these reasons, the effective SPF of a sunscreen product often actually deviates from the labeled SPF. Therefore, a solution is needed for dynamic calculation the effective SPF of the applied sunscreen. This is important for at least two reasons. First, it allows more accurate estimation of the accumulated UV dose received by the subject, thus can help user to better protect against skin damage caused by over exposure of UV radiance. Second, since a high SPF sunscreen dramatically reduces the skin's capability to synthesize vitamin D, knowing the effective SPF allows more accurate estimation of the vitamin D production.
Exposed Body Surface Area
Because only skin surface exposed to sunlight can synthesize vitamin D, different clothing choices can significantly affect the amount of vitamin D production in a person. Therefore, to estimate the amount of cutaneous vitamin D synthesis based on the UV dosage, it is important to know the percentage of exposed body surface area, taking into account of various options of clothing coverage. Consequently, there is a need for a solution to efficiently calculate the exposed body surface area corresponding to common clothing choices in order to accurately measure a user's vitamin D production.
Reliable UV Sensor Measurement
Yet another challenge is to reject or correct unreliable UV sensor measurement. If the UV sensor is not aligned with the direct solar irradiance, the sensor measurement may underestimate the UV intensity at surfaces that are normal to the sunlight. In particular, the angle sensitivity of UV sensor with respect to sunlight poses a practical challenge for a wearable UV dosimeter, since its orientation can change anytime as the user varies position and/or posture while the sun irradiance angle also varies constantly. Even subtle change of sensor orientation and/or the shade coverage caused by user movement can dramatically change the sensor measurement. Thus the direct measurement from the wearable UV dosimeter may be less than the actual UV intensity on the user's body surface that is normal to the sun light. On the other hand, when the subject moves to indoor or into the shade, the low UV intensity measured by the UV sensor indeed correctly reflects the actual UV intensity on user's body surface. Therefore, in order to more accurately estimate the UV dosage received by the user, a solution is needed to determine if measured UV intensity data should be corrected, and filter out the unreliable UV sensor measurement data if such correction is needed.
Sun Exposure Time
Solar radiation has been used since ancient times to treat various diseases. Although the beneficial effects of solar radiation are mainly mediated via UV induced vitamin D production in skin, several other pathways may exist for the action of UV radiation on humans. In particular, exposure to sunlight has mood enhancing effect, which is believed to be mediated through the release of serotonin. Research has shown that the rate of production of serotonin by the brain was directly related to the prevailing duration of bright sunlight, and rose rapidly with increased luminosity. The concept—that sunlight has beneficial effects on the serotonin system and the mood and stability of humans—is consistent with the idea that serotonin is involved in homeostasis in humans and contributes to the emergence of mind. Therefore, there is a need for a solution to calculate the daily sun exposure time (SET), thus allowing user to potentially correlate SET with her/his mood of the day. Such a feature may not only reveal the likely correlation between mood and SET, but also provides the user guidance to enhance mood by consciously adjusting SET.
Prediction of Optimal Sun Exposure Time
While it is helpful for a person to obtain instant feedback on received UV dose and cutaneous vitamin D production and use such information as a guidance to optimize sun exposure, more benefits can be gained if the person knows in advance how to spend time outdoors in the near future for optimal sun experience. For example, it would be very helpful if a person knows what time and how long to stay outside in the next few days to avoid sun damage and to generate sufficient vitamin D. Therefore, there is a need for a solution to provide the user predictive information on optimal sun exposure in the near future, so that the user can plan ahead based on such information.